Technical Field
The present invention relates to an image processing device to eliminate noise superimposed on a radiation image and particularly relates to the image processing device so as to improve quality of the image of each frame forming a live image.
A medical facility equips a radiation imaging device to acquire the radiation image. Some radiographic devices can continuously image the radiation image and then output the results as a video. Such video can be called as a live image (e.g., refer to Patent Document 2).
The live image is obtained by irradiating a low-dose radiation to a subject, so that the S/N ratio thereof is inferior to the S/N ratio for static imaging (spot imaging) and a lot of noises can be included therein. A radiographic imaging device equips an image processing device that reduces such noise. Such image processing device can structurally provide the noise reduction image having the S/N ratio improved by superimposing multiple continuous frames with time.
A recursive filter is applied as a method to generate a noise reduction image. According to the recursive filter, a noise reduction image formed by superimposing the live image frame each other is stored in memory to be as a superimposition target image. And when a new live image frame is input, the new live image and the superimposition target image are superimposed to form the new noise reduction image that is also stored in addition. Since then, every time when the new frame is input, an operation to superimpose the stored noise reduction images each other is executed and generates the noise reduction image corresponding to the frame one after the other. Such recursive filter operation may be called as a circuit addition process.
Meanwhile, the live image is a video catching the movement of the subject. Accordingly, if the images are simply superimposed each other, a noise reduction image of which the subject images are as if doubled. Because the imaged position and shape of the subject are different each other between two images. In addition, since the level of misalignment between the subject's images per se varies depending on the portion of the image, even if one frame is just shifted relative to another frame to superimpose, doubling of the images cannot be prevented. The subject image of the live image has a portion at which the images between frames are the same each other and a portion at which the images do not coincide. The same portions are simply just superimposed to provide noise reduction. However, the different portions provide a misaligned image of the subject.
Therefore, according to the conventional aspects, a variety of devices is adopted to prevent such image doubling. For example, according to the aspect of Patent Document 1, the superimposing aspect is changed every pixel forming an image. Specifically, the portion in which the variation of the pixel values between the frame and the superimposition target image is sever (images do not coincide) is processed to succeed the pixel value of the frame as-is. If such portions are simply superimposed, the images are most likely doubled. And the portion in which the variation of the pixel values between the frame and the superimposition target image is not much (images coincide) is processed to superimpose the frame and the superimposition target image.
In addition, according to the aspect of Patent Document 2, the aspect of superimposing is changed every pixel considering not only the difference of pixel values between images but also an amount of noise imaged into the image. Accordingly, the area in which more noise in the frame is imaged is selectively subject to the noise reduction process. Then, preventing doubling images and reducing the noise can be brought in reality at the same time.
And according to the aspect of Patent Document 3, the place of the superimposition target image, corresponding to each pixel in the frame, is examined by an image analysis and then an operation to superimpose the pixel in the frame and the corresponding pixel in the superimposition target image thereto are executed. Accordingly, the structure in the frame and the structure in the superimposition target image can be superimposed while aligning each position. Accordingly, it has been considered that prevention of doubling images and reduction of noise should be surely performed.